Fly ash is the solid material which is carried away from the power plant boiler in the flue gas during coal combustion. The properties of fly ash may vary considerably according to several factors such as the geographical origin of the source coal, conditions during combustion, and sampling position within the power plant. A typical aggregate of fly ash from the combustion of eastern U.S. coals consists of spherical particles embedded in an amorphous matrix. Most fly ash particles are in the silt-sized range of 2-50 microns. The three major mineralogical matrices identified in fly ash are glass, mullite-quartz, and magnetic spinel. The major elemental constituents of fly ash are Si, Al, Fe, Ca, C, Mg, K, Na, S, Ti, P, and Mn. Nearly all naturally occurring elements can be found in fly ash in trace quantities. Certain trace elements, including As, Mo, Se, Cd, and Zn, are primarily associated with particle surfaces. The solubility of fly ash has been extensively investigated. Results of these investigations are largely dependent on factors specific to the extraction procedure. The most abundant species in fly ash extracts are inorganic ions derived from Ca, Na, Mg, K, Fe, S, and C. Boron is much more soluble than other trace elements in fly ash. The forms of some elements in fly ash extracts have been determined, but the species of most trace elements remain unidentified. Long-term leaching studies predict that fly ash will lose substantial amounts of soluble salts over time, but simulation models predict that the loss of trace elements from fly ash deposits through leaching will be very slow. The constituents of coal fly ash include small amounts of radioisotopes which do not appear to be hazardous. A complex mixture of organic compounds is also associated with fly ash particles. The organic compounds identified in fly ash extracts include known mutagens and carcinogens. Better methods for the extraction of organic compounds from fly ash particles must be developed before these compounds can be fully identified and quantified.(ABSTRACT TRUNCATED AT 400 WORDS)